Surveying Africa’s largest uranium mine

Extra tight deadlines, harsh weather conditions and the biggest uranium mine in Africa – these were the project conditions that met Strydom and Associates in the Namib Desert. Using an unmanned aerial vehicle (UAV), the surveyors were able to conduct the needed volumetric measurement over a 15 km2 open pit mine in a single day.

For a country that barely occupies 0,16% of the Earth’s surface, Namibia produces a significant share (10%) of world’s uranium. Not only does this make Namibia the fourth largest uranium producer in the world1, uranium mining itself contributes to more than 10% of Namibia’s GDP. In the coming years, this contribution to GDP is expected to increase as the Husab Mine reaches its full production capacity. The mine, located 50 km south-east of Swakopmund, has the potential to produce 6800 t of uranium oxide per annum and contains approximately 280-million tonnes of uranium ore. This makes it Africa’s largest, and the world’s second largest open pit uranium mine.

Fig. 1: Husab Mine in the Namib Desert is to be the largest uranium mine in Africa at full production.

As 2017 drew to a close, the task of surveying the main pit and stockpiles of this mine fell on Strydom and Associates’ shoulders. The company had to determine and submit volumetric measurements for the year-end audit before 3 January 2018.

Strydom and Associates, using a multidisciplinary approach, have completed diverse projects ranging from a topographical survey of 0,5 ha to an aerial survey of 10 000 ha, with their clients spread across southern Africa. However, to keep the tight deadline for the Husab Mine project, they needed to map a large area in a very short time at a high accuracy.

Combining fixed-wing and multirotor benefits

Beyond the project constraints, there were environmental challenges to account for. The Namib Desert’s hostile climate makes Namibia the second least densely populated country in the world. The temperature and precipitation fluctuate widely and harsh desert wind – also called East Wind – can reach speeds of up to 30 m/s.

Fig. 2: The high-resolution imagery produced by the 42 MP RX1RII camera with a low distortion 35 mm lens.

Given the magnitude of the project, the tight deadline and the adverse environmental conditions, the surveyors ruled out ground-based surveying methods as these can be time consuming. They also rejected the idea of using manned aircrafts because of accuracy limitations, much higher costs and the very same timing constraint. Aerial surveying was their best option. With a UAV, they could collect all the required data in a matter of few hours at a fraction of the cost.

Fig. 3: The WingtraOne is a VTOL UAV that takes-off and lands vertically like a multirotor and transitions into flying like a fixed-wing aircraft.

However, not every professional mapping UAV can deliver high accuracy results while covering large areas. There is also the choice between fixed-wings and the multirotors. Multirotor craft automatically gets ruled out as they are not typically adept to cover vast areas. On the other hand, fixed-wings can be difficult to land on the complicated terrain as they perform belly landings, which means that they basically slide on the ground posing a threat to the onboard cameras.

The surveyors thus turned to use WingtraOne’s vertical take-off and landing (VTOL) UAV. This UAV takes-off and lands vertically like a multirotor, but in-flight transitions into flying like a fixed-wing aircraft. VTOL combines the benefits of each type of craft: as a fixed-wing, it can deliver large coverage, and as a multirotor, it can take off and land anywhere without damaging its high-resolution cameras.

To obtain high accuracy results with aerial surveying, the presence of ground control points and the use of a high-resolution camera was important. The ground control points already existed in the area and so did not need any additional time. The surveyors used the WingtraOne UAV equipped with a high-resolution camera, the 42 MP full-frame Sony RX1RII.

Mapping large areas in short times

Fig. 4: A sample from the resulting digital elevation model (DEM) used for volumetric calculations. Details such as dimples caused by the blast holes are visible.

On 31 December 2017, equipped with the UAV, the survey team set out to map the 15 km2 area. Flight planning was done using the WingtraOne’s custom flight planning software, WingtraPilot. The surveyors had already outlined the area they wanted to map on the software’s base layer map, and it had generated the required flight plans. Four flights were planned in total, each one set to a GSD of 5 cm/px, requiring a flight altitude of 390 m.

At site, the ground wind speed was 7 m/s. The software ran a number of automated safety checks before the flight to make sure that operations will be undertaken safely. This and additional mission planning features on the app allow the surveyors make minor adjustments to the flight plans and allow complete hands-off safe operation.

For each flight, the craft automatically took-off, flew along the generated flight path capturing images and then landed safely on its 4 m2 landing spot. In 2,5 hours of flight time, the surveyors collected aerial imagery consisting of 1500 RGB images of the entire 15 km2 area. The images were georeferenced using the onboard GPS data from the UAV.

The georeferenced images were post-processed in Agisoft Photoscan software. The evenly distributed ground control points in the area helped to optimise camera positions and orientation data. Combining the overlapping images and location data, highly accurate 3D models were built. At the end, the team was able to achieve 1 cm relative accuracy both horizontally as well as vertically in their models, which is important for volumetric measurements.

Meeting challenging demands

Fig. 5: A 3D view of part of the contour map.

Ease of flight planning means that small last-minute changes to the flight plans designed in the office could be made to meet the conditions experienced on site. Short set up times, hands-off operation and flexible planning allowed the team to complete their task of data collection on the field efficiently.

The main advantage with the WingtraOne UAV was the high-quality images from the camera. The Sony RX1RII camera with its 35 mm lens makes it possible to cover the area efficiently at a altitude of 390 m. This efficiency helped meet the deadline. The level of detail afforded by the full frame camera meant that further analysis in the office could also be completed quickly and with high accuracy.

Strydom and Associates could keep their deadline and compile a set of valuable data in this harsh environment. The VTOL UAV was a good choice for its reliability in helping the surveyors conduct an end-of-year audit survey of the Husab Mine over the leap from 2017 to 2018.